Power exhausters or vents are typically mounted to building roofs either vertically or in line with the slope of the roof. These exhausts and vents are usually in a normally closed position and do not open until a powered exhaust fan is activated or hot air increases the atmospheric pressure to the point that the dampers or louvers open. Upon a drop in the pressure torsional springs urge the open louvers to a closed position. Such devices require expensive motors and/or fans and normally move the louvers only between discrete closed end fully opened positions.